Image pickup apparatuses such as digital cameras can produce an image signal having a proper signal level by adjusting the exposure time automatically or manually in accordance with the brightness of an object.
FIG. 1 is a block diagram showing the configuration of a conventional image signal processing apparatus for processing an image signal that is generated by an image pickup device. An image signal that is output from an image pickup device 2 such as a CCD (charge-coupled device) image sensor is processed by an analog signal processing circuit 4, converted to digital data by an A/D conversion circuit 6, and input to a digital signal processing circuit 8. The digital signal processing circuit 8 is equipped with an LPF (low-pass filter) 10 as a filter for eliminating noise that may cause moire. The LPF 10 traps a frequency component whose frequency is ½ of a sampling frequency in each of the vertical and horizontal directions. An output of the LPF 10 is subjected to gradation correction processing in a gamma correction circuit 12. In an automatic exposure control operation, the digital signal processing circuit 8 determines an integration value of a one-frame image signal and feedback-controls the exposure time so that the integration value comes to have a proper level.
An outline correction circuit 14 performs outline emphasis processing that is one of image quality adjustments. The principle of the outline emphasis processing is such that a second derivative waveform of an image signal that varies very much at edge portions of an image is determined, multiplied by a proper gain, and then combined with the original image signal.
The exposure time of the image pickup device is set longer as an object becomes darker. And there is a problem that the shot noise due to a dark current becomes higher as the exposure time becomes longer. Further, in the outline emphasis processing, the rise and fall of each shot noise are also detected and subjected to the outline emphasis. That is, where the outline emphasis is performed in the above manner, shot noise is enhanced in an image that is displayed on the screen. Marked deterioration in image quality is caused by enhanced shot noise due to an increased exposure time.